J=lambda:map(int,input().split())
N,M=J()
A=list(J())
b=[0]*(N+1)
S=[]
for s in J():
	b[s]=1
	S+=[s]
D=S[:]
for _ in range(N):
	E=[]
	for i in S:
		for j in D:
			t=A[i+j]
			if b[t]<1:
				b[t]=1
				E+=[t]
	D=E
print(sum(b))